The present invention relates generally to a satellite communication system capable of multicast processing, and more particularly, relates to such a system in which the capacity of the multicast processing can be scaled to meet different requirements.
Modern communications networks carry staggering amounts of information. Increasingly, that information is transmitted through communication satellites. A single satellite may have, for example, the equivalent of 30 or more uplink transponders, each able to receive an uplink signal with a bandwidth of 250 MHz. The resultant uplink data path may have a capacity of 8 to 60 gigabits per second or more.
Where a satellite is a link in a communications network, many individual ground stations may encode, modulate, and transmit uplink signals to the satellite. Each uplink signal may consist of hundreds of individual data channels each, for example, carrying data for a telephone conversation. Similarly, the downlink signals produced by the satellite and transmitted to ground stations often include data for hundreds of users.
Additionally, crosslink signals may transmit data between satellites.
Satellite (and terrestrial) communication systems divide the data traffic on the uplink, downlink, and crosslink signals into discrete pieces of information, and each discrete piece of information may subsequently be transmitted over different selected channels in the communication network. The discrete pieces of information are referred to, for example, as xe2x80x9cframesxe2x80x9d or xe2x80x9cATM packetsxe2x80x9d, depending on the particular system. In past terrestrial systems, for example, the data packets may be Asynchronous Transfer Mode (ATM) cells.
Each ATM cell is a specifically formatted data packet that is 53 bytes long and includes 5 header bytes (called the xe2x80x9cheaderxe2x80x9d) and 48 information bytes (referred to below as the xe2x80x9cinformation payloadxe2x80x9d). The header contains necessary information for a network to transfer cells between nodes over an ATM connection.
Specifically, the header contains a logical address consisting of an 8-bit Virtual Path Identifier (VPI) and a 16-bit Virtual Channel Identifier (VCI). The header also contains a 4-bit Generic Flow Control (GFC), a 3-bit Payload type (PT), and a 1-bit Cell Loss Priority (CLP) indicator. The header is error-protected by a 1-byte header error check (HEC) field.
The VPI/VCI field of an ATM header cell contains ATM address information. A virtual channel is used for the unidirectional transport of ATM cells, each channel having associated with it a VCI value. A virtual path (VP) is an aggregate bundle of virtual channels (VCs). These paths have associated VPI values, each VPI value identifying a bundle of one or more VCs. Because two different VCs belonging to two different VPs at a given interface may have the same VCI value, a VC is only fully identified at an interface if both its VPI and VCI values are indicated. Thus, the ATM address field is divided into two subfields. The first byte contains the VPI. The information in this field is used to switch virtual paths consisting of groups of virtual channels. The second byte contains the VCI, used to switch virtual channels. The information in the VCI identifies a single virtual channel on a particular virtual path.
Connection to an ATM network is a shared responsibility. The user and network provider must agree as to the support of application bandwidth demands and other traffic characteristics that will be provided. To further one aim of such an agreement, that is assurance that the integrity of the transmitted data packets is maintained, network users categorize cells according to Quality of Service (QoS) classes. QoS is defined by specific parameters for cells that conform to a particular traffic application. Traffic parameters are negotiated between a user and a network provider, and the user""s input cells are monitored to ensure that the negotiated traffic parameters are not violated these parameters can be directly observed and measured by the user. QoS is defined on an end-to-end basis, an end, for example, being an end workstation, a customer-premises network, or a private or public ATM user-to-network. QoS is defined in terms of any number of measurement outcomes.
The measurement outcomes used to defined ATM performance parameters include successful cell transfer, errored cell transfer, lost cell, misinserted cell, and severely errored cell block. These performance parameters correspond to the generic QoS criteria of accuracy, dependability, and speed. Measurements of cell error ratio, severely errored cell block ratio, and cell misinsertion rate correspond to the QoS criterion of accuracy; measurements of cell loss ratio correspond to the QoS criterion of dependability; and measurements of cell transfer delay, mean cell transfer delay, and cell delay variation correspond to the QoS criterion of speed.
Cell error ratio (CER) is defined as the number of errored cells divided by the sum of successfully transferred cells and errored cells. Severely errored cell block ratio is defined by the total transmitted cell blocks. Cell misinsertion rate (CMR) is the number of misinserted cells divided by a specified time interval. Cell loss ration (CLR) is defined as lost cells divided by transmitted cells. Cell transfer delay is the elapsed time between a cell exit event at a measurement point and a corresponding cell entry event at another measurement point for a particular connection.
The cell transfer delay between two measurement points is the sum of the total inter-ATM node transmission delay and the total ATM node processing delay between the measurement points. Mean cell transfer delay is the average of a specified number of absolute cell transfer delay estimates for one or more connections. Cell delay variation is a measure of cell clumping, i.e., how much more closely cells are spaced than the nominal interval. Cell clumping is an issue because if too many cells arrive too closely together, cell buffers may overflow.
QoS classes are defined with pre-specified parameter threshold values. Each QoS class provides performance to an ATM virtual connection as dictated by a subset of ATM performance parameters. Additional details on ATM cell headers and QoS classes may be found in numerous references including ATM Theory and Application, (David E. McDysan and Darren L. Spohn, McGraw-Hill, Inc. 1995).
The use of QoS classes for ATM switches assures the integrity of the data packets. In addition, in most applications (where capacity generates revenue), one significant performance factor is the amount of information that is passed through the communication system (i.e., throughput). Generally, the higher the data throughput, the higher the revenue potential.
In the past, a bar to implementing a high throughput space-based switch was that an earth terminal could only receive information in a particularly configured downlink at any given moment in time. The downlink configuration depends on several parameters including, for example, frequency, coding, and the polarization of the downlink at the time the satellite transmits the information. Unlike information transmitted terrestrially, not every earth terminal may receive information in every downlink, because earth terminals are only configured to receive a particularly configured downlink at any given moment in time.
Thus, space-based systems, unlike terrestrial systems, face unique challenges in their delivery of information to ground stations. In other words, past terrestrial networks did not provide a suitable infrastructure for communications satellites.
Furthermore, revenue generated from operation of satellite communication systems is affected by considerations of weight and power consumption of the switch used in the satellite communication system. In a space-based implementation, higher weight and increased power consumption in the switch translates to higher spacecraft and launch costs and potential for reduced throughput. These, in turn, may have the effect of lowering potential revenue. Thus, cell switch features which, when implemented, minimize weight and power consumption of the switch are desirable because such added features functionality do not adversely affect the bottom line.
One such feature is that of providing a multicast capability. The use of multicast in a cell switch allows sending the same data to a selectable number of destinations. This feature addresses inefficient use of bandwidth in terrestrial switches. Prior to multicast feature availability, a broadcast function was commonly used, which sent the same data to all possible destinations. With the implementation of multicast, only those destinations requiring the data received it, making unused bandwidth available for data packet transmission.
Multicast capability as implemented in terrestrial based switches is inappropriate for use in satellite communications systems, however, due to its cost, weight, and complexity. Nonetheless, the need for multicast functionality in spaced-based cell switches is particularly important because if available, it would serve to maximize the efficient use of uplink, downlink and crosslink bandwidth.
Without on-board satellite multicast capability, replication must be performed on the ground, with multiple copies sent through the uplink. Thus, because a source terminal is required to send the same data repeatedly using this replication strategy, uplink bandwidth is wasted. Similarly, with only a broadcast capability in the space-based switch, data would be sent to all downlink and crosslink beams, resulting in wasted downlink and crosslink bandwidth. Multicast capability in a space-based switch would allow data to be sent only to those destinations requiring it, thus optimizing bandwidth. Whether used in terrestrial or space-based systems, multicast ideally supports modern network standards, such as ATM and ATM""s QoS parameters (as discussed above).
Depending on the target business plan, the user may choose to implement the multicast capability in a scalable and modular fashion as a trade to having more or less dedicated uplink and downlink bandwidth.
One aspect affecting weight and power of a cell switch is the number of data packet storage units, for example, queue buffers, used to implement the multicast feature. In the past, multicast has been implemented using multiple queue buffers (e.g., one on the in-bound side of the switch, and one on the outbound side). The queue buffers store ATM cells until they are extracted for subsequent processing and eventual routing to specified destinations. This multiple queue buffer implementation is designed to support ATM QoS parameters. Though a multiple buffer implementation may be efficient for the terrestrial switches, this approach is undesirable in spaced-based switches. This is so because such an implementation unnecessarily increases the weight and power consumption of the switch. Increased weight and power translates into higher spacecraft and launch costs and reduced throughput, which can translate into a loss of revenue. Additionally, increased complexity may result in lower reliability.
Terrestrial communication networks have been moving in recent years towards ATM standards. Often, it is desirable to link the terrestrial communication networks through a satellite system. In the past, however, there has been no efficient multicast capability available for space-based implementation, thus barring the progress of globally providing reliable and economic information transfer.
A need exists in the satellite communication industry for efficient multicast capability in a commercial satellite system using a scalable and modular multicast architecture such that an appropriate number of multicast modules may be employed to satisfy the requisite multicast capacity.
It is the object of the present invention to provide a satellite multicast switching architecture that is scalable and modular to uniquely satisfy the need of different satellite systems. The multicast capacity of each system is based on their projected business and revenue plans.
It is also an object of the present invention to provide a satellite multicast switching technique with the ability to adapt a common design to keep non-recurring engineering costs to a minimum.
According to a preferred embodiment of the present invention, a switch is provided for receiving data cells at a set of input ports. The received data cells which include a multicast routing code are switched to a first set of output ports or nodes and received data cells which effectively omit a multicast routing code are switched to al second set of output ports or nodes. The data cells switched to the first set of output ports or nodes are copied or replicated, and the multicast routing code is effectively omitted from the replicated data cells which are then coupled to the input ports so that the replicated data cells are transmitted to the second set of output ports or nodes of the switch means. Outbound processing is performed on the data cells coupled to the second set of output ports or nodes in order to prepare the data cells for transmission, typically by a downlink of a satellite.
According to a modification of the previous embodiment, inbound processing is also provided whereby the multicast routing code is written into the data cells in the satellite before they are transmitted to the switch means.